1. Field of the Invention
The present invention relates to a track jumping apparatus for an optical disk pickup for jumping a light beam from the optical disk pickup over tracks on an optical disk.
2. Description of the Prior Art
The track jumping apparatus for an optical disk pickup moves a light beam between tracks by sending a jump signal S.sub.2, an exemplary waveform of which is shown in FIG. 7, to a tracking actuator. Also, a tracking servo loop is opened during this track jump by turning a gate control signal S.sub.3 to "LOW". An exemplary waveform of S.sub.3 is also shown in FIG. 7.
This jump signal S.sub.2 accelerates the tracking actuator by applying a pulse of +V.sub.1 volt for a time T, and subsequently decelerates and stops the tracking actuator by applying a pulse of -V.sub.1 volt for the same time T, and thus the light beam can be moved by one track at a time during this operation. Accordingly, the quantity of a tracking error signal S.sub.1 is increased to one side as the light beam parts from the track. However, when the light beam exceeds the center line of the space between tracks, it approaches an adjacent track from the reverse side, and therefore the quantity of error is inverted abruptly, being increased on the other side. Then, as the light beam further approaches the adjacent track, the quantity of error is decreased to a greater extent, and finally, when the quantity reaches zero, the track jump is completed.
Examples of such waveforms S.sub.1, S.sub.2, and S.sub.3 and the manner in which they change over time in accordance with prior art track jumping devices is shown in FIG. 7.
Where the time of acceleration or deceleration of the jump signal is T.sub.1, as shown, for example by a solid line the gate control signal S.sub.3 thereafter returns to "HIGH" and closes the tracking servo loop, and the tracking error signal S.sub.2 soon converges towards zero. Consequently, the light beam quickly stops on nearly the adjacent track, and therefore a stable track jump can be performed. Then, when the time of acceleration or deceleration is set to T.sub.2 which is longer than T.sub.1, as shown by a broken line in FIG. 7, the light beam overshoots the adjacent track to a large extent at the end of deceleration of the tracking actuator, and therefore the tracking error signal S.sub.1 has not returned to zero after the gate control signal S.sub.3 has closed the tracking servo loop, and a period of time is subsequently required to stabilize the tracking servo. Also, when the time of acceleration or deceleration is excessively short, the tracking servo loop is closed before the light beam fully reaches the adjacent track, and likewise a period of time is required to stabilize the tracking servo. This means that in performing a track jump, an optimum time for acceleration or deceleration must be determined.
However, f the time T of acceleration or deceleration of the jump signal is fixed in advance to a constant time whereby an optimum track jump is obtained, a stable track jump cannot be obtained where a track pitch is not constant, or where a change takes place in the characteristics of the tracking actuator.
For this reason, in an invention described in the Japanese Published Unexamined Patent Application No. SHO 59(1984)-84379, overshooting or undershooting of a jump is determined by the state of the tracking error signal S.sub.1 at the end of each track jump, and thereby the time T of acceleration or deceleration of the subsequent jump is adjusted accordingly. However, a device incorporating such a methodology is effective if the length of time to make the adjustments is short in comparison with the jump time, but cannot accommodate conditions where rapid changes must occur over relatively short periods of time.
Also, in the Japanese Published Examined Patent Application No. SHO 57(1982)-1051, an invention has been disclosed wherein the time T of acceleration or deceleration is adjusted based on a comparison of the tracking error signal S.sub.1 during track jump to a reference level V.sub.R. The method taught by this reference, also shown in FIG. 7, compares the tracking error signals S.sub.1 with a reference level V.sub.R1, and changes the jump signal S.sub.2 to the deceleration signal upon intersection with the reference level V.sub.R1 at which time the quantity of error is inverted between the tracks, and thereby obtains an optimum time T.sub.1 of acceleration or deceleration. For example, when this reference level V.sub.R1 is set at a lower level to V.sub.R2, as shown by a broken line in FIG. 7, the timing of the changeover of the jump signals S.sub.2 is delayed, and the time of acceleration or deceleration is extended to as long as T.sub.2, and as described above, the light beam jumps an amount which overshoots the target track. Accordingly, by setting a suitable reference level V.sub.R in advance, the optimum time T of acceleration or deceleration is obtainable, and the apparatus can accommodate for expansion and contraction of the tracking error signal S.sub.1 on the time axis.
Also, real-time control is made possible, and therefore a stable track jump can be performed even when a short period of change is required. However, as taught by the aforementioned publication, when a level change takes place in the tracking error signal S.sub.1, the relationship between the waveform of this tracking error signal S.sub.1 and the reference level V.sub.R is not kept constant, and a stable track jump cannot be performed.
Then, conventionally, an invention has been proposed in the Japanese Published Unexamined Patent Application No. SHO 61(1986)-276134 wherein in comparing the tracking error signal S.sub.1 during a track jump with the reference level V.sub.R, the time T of acceleration or deceleration is adjusted in view of the peak value thereof.
As in the case of adjusting the time T of acceleration or deceleration based on the comparison of the above-mentioned tracking error signal S.sub.1 with the reference level V.sub.R, the track jumping apparatus of the previous publication changes the jump signal S.sub.2 to the deceleration signal when the tracking error signal S.sub.1 intersects the reference level V.sub.R, and thereby obtains an optimum time T of acceleration or deceleration. These waveforms are illustrated graphically in FIG. 8. Note that, at this optimum time T in view of the level change in the tracking error signal S.sub.1, the reference level V.sub.R is set so that the ratio of the reference level V.sub.R to a peak value V.sub.P of this tracking error signal S.sub.1 is constant.
This means that, as shown by the waveforms and lines shown in FIG. 8, taking a reference level for a peak value V.sub.P1 of the standard tracking error signal S.sub.1, when a stable track jump is performed as V.sub.R1, where the actual tracking error signal S.sub.1 is reduced in level as shown by the broken line, a reference level V.sub.R2 is set so as to maintain the relation V.sub.R1 /V.sub.P1 =V.sub.R2 /V.sub.P2 for a peak value V.sub.P2 at that time. Accordingly, the jump signal S.sub.2 maintains a constant time T.sub.1 of acceleration or deceleration even when such a level change takes place and therefore a stable track jump is nonetheless performed.
However, such a tracking jumping apparatus has a problem in that an optimum time T of acceleration or deceleration cannot be obtained when there is an off-set (asymmetry) which takes place in the tracking error signal S.sub.1. An off-set series of waveforms is also shown in FIG. 9.
The off-set condition shown in FIG. 9 occurs when an inversion for the quantity of error is made at an early timing as shown by a broken line compared with the standard tracking error S.sub.1 which is shown by a solid line. Also in this case, the track jumping apparatus of the previous publication sets the reference level V.sub.R2 for the peak value V.sub.P2 so that V.sub.R2 /V.sub.P2 is equal to V.sub.R1 /V.sub.P1 in the instance where the tracking error signal S.sub.1 is shown by a solid line, and therefore the time T of acceleration or deceleration becomes T.sub.2 which is shorter than T.sub.1 the optimum which was determined originally, and a full track jump is not performed, and the apparatus undershoots the jump between tracks. Accordingly, if the servo loop is closed when the quantity of error of the tracking error signal S.sub.1 is still large, a time period is required to recover the tracking. It is obvious that also in the instance where an off-set of the delayed inversion has taken place in the tracking error signal S.sub.1, time is required to recover the tracking because of overshooting the jump between tracks.
In performing the actual track jump, for example, the jump to an inner circumferential side and the jump to an outer circumferential side in the case of a circular optical disk storage device differ from each other in the waveform patterns formed, and particularly at the position of inversion of the standard tracking error signal S.sub.1. In addition, the above description primarily concerns the case where the jump is over only one single track. However, in the case where jumping occurs over multiple tracks, each standard tracking error signal S.sub.1 also differs in its wave-form pattern.
Accordingly, there has been a problem in trying to determine the time T of acceleration or deceleration using the same base for these various track jumps and optimum control of each jump becomes impossible.
The object of the present invention is to overcome the above-mentioned problems, by providing a track jumping apparatus for an optical disk pickup. which can set the reference level based on the peak value of the tracking error signal using a standard value which varies depending on the kind of track jump, and can amend this standard value according to the state of tracking error signal at the end of track jump.